The present invention relates to a battery device (hereinafter, referred to as "battery pack") including a circuit required to be monitored for a battery state such as a voltage or a charge/discharge current, a battery state monitoring circuit for monitoring the circuit, an external connection terminal outside of the battery device, a switch element, a secondary battery and a sense resistor in a battery device for a secondary battery.
As a conventional battery pack, there has been known a device shown in a circuit block diagram of FIG. 2. For example, Japanese unexamined patent application publication No. H 9-312172 (1997) entitled "Battery pack, charger and charging system as well as charging method" discloses the structure of this type. This is directed to a battery pack which is commonly called a "smart battery system" or the like. That is, a battery pack 120 is structured by a battery state monitoring circuit 18A, a switch element 102a and 102b, a sense resistor 10, a cut out circuit 19 and secondary batteries 6 to 8. The battery state monitoring circuit 18A is structured by a microcomputer 4A, a battery voltage monitoring circuit 20A and an amplifier 3, and has a function of monitoring a voltage and the charge/discharge current of the secondary battery. Such a battery pack 120 can conduct communication with a charger 17, external equipment such as a microcomputer 5, or with a load 16.
Employing the battery pack 120 thus structured, it is possible to recognize a battery state by communication with the charger 17, the microcomputer 5 within a personal computer, a load 16 or the like. The use of this information allows an indication of the residual amount of the battery, a suspension of battery charge, etc., to be conducted accurately.
In the case where a lithium ion battery is used for the secondary battery, since a self-protecting action is not provided unlike a nickel cadmium battery, an over-charge protecting circuit is required. That is, the lithium ion battery is provided with some circuit for detection of a battery voltage and a switch element 102a and 102b for suspending charging operation from outside.